The present invention is directed to control circuits for gas discharge lamps. More specifically, the present invention is directed to a resonant flyback ignitor circuit for igniting gas discharge lamps.
Gas discharge lamps are used in a variety of applications. For example, mercury vapor lamps are used for ultraviolet (UV) curing of ink in printing presses, for curing furniture varnish, in germicide equipment for killing germs in food and its packaging, for killing bacteria in medical operating rooms and for lighting applications such as high intensity discharge (HID) lighting. Many other applications also exist.
A traditional circuit for controlling a mercury vapor lamp includes an AC power source which drives a primary side of a ballast transformer. A secondary side of the transformer is coupled to the lamp. The lamp includes a gas-filled tube with electrodes at each end of the tube. The secondary side of the transformer applies a voltage between the electrodes which accelerates electrons in the tube from one electrode toward the other. The electrons collide with gas atoms to produce positive ions and additional electrons. Since the current applied to the gas discharge lamp is alternating, the electrodes reverse polarity each half cycle.
Collisions between the electrons and the gas atoms generate additional electrons. Therefore, an increase in the arc current causes the impedance of the lamp to decrease. This characteristic is known as "negative resistance." The lamp is unstable, and current between the electrodes must be limited to avoid damaging the lamp. As a result, a typical control circuit includes a current limiting inductance coupled in series with the lamp. The inductance can either be a physically separate inductor or "built-in" to the transformer as a leakage inductance.
When the lamp is first started, the lamp requires a very large striking voltage to initiate an arc to ionize the gas in the lamp. The electrodes of the lamp are cold and there are almost no free electrons in the tube. The impedance of the lamp is therefore very high. The voltage required to initiate the arc exceeds that required to sustain the arc. For example, the ignition voltage may be 1,000 volts while the operating voltage may be 100 volts. In such cases, a device known as an ignitor has been added to the ballast transformer.
A typical ignitor circuit superimposes high voltage spikes on the normal output voltage produced by the secondary side of the ballast transformer. These high voltage spikes do not provide significant power themselves, but overcome a potential barrier that would otherwise prevent ionization of the plasma in the lamp during each half-cycle of the AC power being delivered to the lamp.
The high voltage ignitor pulses are typically necessary only during the initial ionization and the warm-up period of the lamp. Once the lamp is at its full operating temperature and power, the ignitor pulses are no longer necessary. Most modern ignitors have timers or are biased such that the ignitors become disabled after a certain time period which is determined to be long enough to fully warm-up the lamp.
In one typical igniter circuit, a resistor-capacitor circuit is coupled to the secondary side of the ballast transformer. Before the lamp ignites, the output voltage of the ballast transformer is sinusoidal like the AC voltage applied to the primary side of the ballast transformer. This voltage appears across the resistor-capacitor network. As the voltage rises, more current passes through the resistor, thereby charging the capacitor. The capacitor continues to charge until the voltage across the capacitor reaches a threshold voltage of a bilateral trigger device. At this point, the bilateral trigger device turns on and applies the capacitor across a small portion of the secondary winding. Through transformer action, the voltage on the capacitor is multiplied by the turns ratio in the winding, and a high voltage appears at the output terminals of the ballast transformer.
Since the energy stored in the capacitor is relatively small, and because the transformer is not designed to support large volt-second values, the igniter output appears as a narrow pulse of high voltage on top of the normal output voltage of the ballast transformer. Each pulse usually lasts only a few microseconds. This type of igniter circuit is typically designed to apply several high voltage pulses per half cycle in order to get the lamp ignited. When the lamp does ignite, the lamp clamps the ballast output voltage to a lower value, which thereby limits the amount of charge supplied to the capacitor. Thereafter, the capacitor never reaches the threshold voltage of the bilateral trigger device. This effectively shuts-off the ignitor after the lamp has ignited.
This type of igniter circuit has several disadvantages for gas discharge control circuits that use modulation or phase control to operate the power delivered to the lamp. These circuits require the lamp current to be reliably initiated. Since the energy per pulse is low in conventional igniter circuits, timely lamp ignition is not ensured. Uncertainty in the ignition timing can result in flickering, instability or loss of control of the lamp current. In addition, several voltage pulses are required in succession for reliable ignition. In order to increase the duration of each igniter pulse to ensure ignition, a different approach would be required along with a larger value storage capacitor. Also, the short, low-energy voltage pulses do not propagate well and are therefore limited to applications in which there is a short distance between the igniter circuit and the lamp. Improved ignitor circuits are therefore desired.